Alloys, dissolution current

The current efficiency of alloy dissolution is calculated by the following equation ... 

Figure 6 displays plots of alloy dissolution productivity (P) versus chromium content for two different electrolytes at a current density of 8 A cm-2. The maximum in Curve 2 is associated with an effect of chromium content of the alloy on the current efficiency in Na2SC>4 and NaNC>3 solutions. The variations in the electrochemical equivalent of the alloy and in the current efficiency with an increase in the Cr content have the strongest effect on the dissolution productivity. An increase in the oxidation state of chromium at Cr 15% has a weak effect on the productivity (a small inflection in the Curve 1 for NaCl). The shape of Curve 2 in Fig. 6 for the NaNC>3 solution depends on the current density in the range of low current densities, the productivity (P) of ECM of nickel and nickel-rich alloys increases with the current density, owing to an increase in the current efficiency. 

It will be shown later that the values of icrit, Epp, and ip, which are the important parameters defining the shape of the active-passive type of polarization curve, are important in understanding the corrosion behavior of the alloy. In particular, low values of icrit enhance the ability to place the alloy in the passive state in many environments. For this reason, the maximum that occurs in the curve at B (Fig. 5.4) is frequently referred to as the active peak current density or, in general discussion, as the active peak. It is the limit of the active dissolution current density occurring along the A region of the polarization curve. 

The nature of copper dissolution from CuAu alloys has also been studied. CuAu alloys have been shown to have a surface Au enrichment that actually forms a protective Au layer on the surface. The anodic polarization curve for CuAu alloys is characterized by a critical potential, E, above which extensive Cu dissolution is observed [10]. Below E, a smaller dissolution current arises that is approximately potential-independent. This critical potential depends not only on the alloy composition, but also on the solution composition. STM was used to investigate the mechanism by which copper is selectively dissoluted from a CuAu electrode in solution [11], both above and below the critical potential. At potentials below E, it was found that, as copper dissolutes, vacancies agglomerate on the surface to form voids one atom deep. These voids grow two-dimensionally with increasing Cu dissolution while the second atomic layer remains undisturbed. The fact that the second atomic layer is unchanged suggests that Au atoms from the first layer are filling... 

The enrichment of the slow dissolving component, B, in an alloy surface under simultaneous dissolution conditions may be rationalized by a model of alloy dissolution that is based on the simplifying assumptions (1) that a homogeneous solid solution may be described as a heterogeneous dispersion of atomic dimensions with area fraction (surface mole fraction) X j for component j, and (2) that the alloy components dissolve independently. The partial current density ij of an alloy component j will then be given by ij = i -X j, where i is the current density of the pure metal, j, and for a binary alloy A-B, the total current density of alloy dissolution. 

Refinements of the above volume diffusion concept have been made by a model that includes a contribution of surface-diffusion processes to the dissolution reaction of the more active component at subcritical potentials. By adjustment of different parameters, this model allows for the calculation of current-time transients and concentration-depth profiles of the alloy components [102]. In addition to this, mixed control of the dissolution rate of the more active component by both charge transfer and volume diffusion has been discussed. This case is particularly interesting for short polarization times. The analysis yields, for example, the concentration-depth profile and the surface concentration of the more noble component, c, in dependency on the product ky/(t/D), where is a kinetic factor, t is the polarization time, and D is the interdiffusion coefficient. Moreover, it predicts the occurrence of different time domains in the dissolution current transients [109]. 

Table 2. Dissolution current densities and corrosion potentials ( ,) for aluminium alloy uncoated and coated with TEOS, PTES and PTMS, after immersion for 24 h in aqueous solution 0.05 mol/1 NaCl.

Under sliding conditions, the currents measured during potentiodynamic polarization are in a first approach the sum of two components, namely the current originating from the rubbed area, and the one linked to the non-rubbed area. Under such conditions, the maximum dissolution current, Im, varies with the mean contact pressure and sliding speed. However, these two test parameters do not necessarily affect in the same way the electrochemical behavior of the alloy ... 

Fig. 3. Schematic potentiodynamic polarization curve recorded on a passivating metallic material (Fe-30%Ni alloy) in absence of any sliding Ef = Flade potential, Em = potential at maximum dissolution current Im, Ip = passivation current.

Regarding stability, Greeley and Norskov (2007) calculated that Pt atoms were more stable in a Pt = solute/Co = host system compared to pure Pt, by using DPT to estimate trends in the thermodynamics of binary surface alloy dissolution in acidic media. Franco and co-workers (2009) simulation results also indicated that PtjCo was more stable than Pt, PtCo, or PtCoj for both low currents (0.1 A cm ) and intermediate currents (0.5 A cm ). 

The alloy composition (and microstructure) has strong effects on all the aspects of passivity that have been described above chemical composition and thickness of the passive film, electronic properties, structure, and kinetics of formation. The influence of alloyed elements on the electrochemical characteristics of passive systems can be seen in Fig. 3-16. This is the same current-potential curve as in Fig. 3-1, on which the two major effects of alloyed elements are indicated lowering of the dissolution current in the active region and at the active-passive transition, and broadening of the passive region. A third effect, not illustrated in Fig. 3-16 but which will be discussed later, is the improvement of the resistance of the alloy to passivity breakdown and localized corrosion. For iron-based alloys, these beneficial effects are obtained with chromium, molybdenum, nickel, and nitrogen. 

Figure 5-17. Effect of plastic strain on the anodic dissolution current of a low alloy steel in high temperature water Note that the current increases when the material suffers plastic yield (Combrade and Foucault, 1989).

---